1. Field of the Invention
This invention relates to graphic data display systems and particularly to such systems in which the position of a locator such as a light pen is displayed.
2. Description of the Prior Art
Users of graphic data display systems use locators such as light pens for cathode ray tube display or styli for tablets for drawing on the display area. Engineering drawings generated in this manner are required to be accurate within specified tolerances as the resultant drawing can be used directly to control a manufacturing operation under a so-called CAD/CAM system. One such system is the IBM 7361 Fastdraft System (IBM is a Registered Trade Mark). The IBM 7361 Fastdraft system includes an IBM 3251 display having a keyboard and a light pen, these comprise a drafting workstation. The display screen displays menus and images within a viewing area. When it is needed, a tracking cross for tracking light pen movement is displayed.
In the IBM 3250 Display System because of the time required for calculations, a digital read-out of the distance and direction traversed by a light pen on the refreshed display is not available until the movement of the light pen is complete. The calculation of the distance and angle traversed by the light pen is performed by the host computer after an interrupt to perform the calculation. This interrupt occurs when the light pen reaches its final point. Such a process is too slow to permit the parameters of the light pen position to be continuously displayed during the light pen movement.
The article `An Improved Light Pen Tracking Algorithm Based on a Recursive Digital Filter` by Murray Kesselman--Proceedings of the S.I.D. Vol. 14/2 Second Quarter 1973 p. 52 et seq describes an algorithm for light pen tracking that can be used in a time sharing environment. The algorithm is only used for tracking and not for providing a display of a light pen position.
A different scheme to identify the position of a light pen is proposed in the article `Direct-View Storage Tube Light Pen Attachment` by W. F. Beausoleil and R. A. Linton, IBM Technical Disclosure Bulletin Vol. 21, No. 6, November 1978, p. 2450. The direct view storage tube has circuitry which is used to cause the display screen to be scanned by a series of horizontal lines until a pen hit occurs. The coordinates of the pen are then transmitted to a host computer. No refresh buffer is required in the terminal to provide the scanning function which is performed in the write through mode.
In order to display the pen position using the Beausoleil-Linton scheme the host computer has to calculate the coordinates and transmit the result to the display terminal. As with the IBM 3250 Display System this does not permit a continuously updated display during the movement of the pen.